Electrical equipment such as transformers, reactors, capacitors, circuit breakers, among others, use internally dielectric fluids, also known as dielectric oils, which are used as an insulation and dissipation means of heat generated by the internal components of the apparatus.
The dielectric fluid must be able to perform its functions effectively and reliable as a cooling and insulation means during the service life of the electric apparatus.
Obviously, many of the properties of the dielectric fluid should be considered as necessary to provide the expected cooling and dielectric capacity, according to its application, therefore, depending on them its ability to function effectively and reliably may be affected. These properties may include: dielectric strength, dielectric constant, dissipation factor, viscosity, acid number, pour point temperature and ignition temperature.
The dielectric strength of the fluid shows its ability to resist electric breaks at certain frequencies of electrical power and it is measured as the minimum electric voltage required to cause the formation of an arc between two electrodes submerged in the dielectric fluid.
The dielectric constant is the ratio of capacitance of a capacitor with a dielectric (oil) between its plates, and the capacitance of said capacitor where the dielectric is a vacuum. This property is related to the ability of the dielectric fluid to conduct electricity, so that at a lower ability value the capacity of the dielectric fluid will increase.
The dissipation factor of a dielectric fluid is the measurement of dielectric losses in this fluid, usually in the form of heat. A low dissipation factor indicates low dielectric losses and a low concentration of polar soluble contaminants in the dielectric fluid.
The acid number in a dielectric fluid is a measure of the constituents or acidic contaminants of the fluid. The acidity of a dielectric fluid is due to the formation of acid oxidation products. Acids and other oxidation products, together with water and solid contaminants affect the dielectric properties and other properties of the dielectric fluid. The rate of increase in acid number of the dielectric fluid is a good indicator of the rate of aging of said fluid.
Pour point temperature is the temperature at which a fluid flow stops, converting into a solid state under environmental pressure conditions. The pour point temperature can also be defined as the maximum temperature (usually below zero Centigrades) for which the dielectric fluid cannot move or deform with its own weight. The lower this temperature, the easier this dielectric fluid shall be applicable in electrical apparatuses subject to extreme low temperatures below zero ° C.
The ignition temperature, also known as flash point, is that temperature the dielectric fluid must reach to result in the ignition of vapors from said fluid, when exposing them to the air and to the source of ignition.
As dielectric fluids cool off by convection, the electrical equipment where they are applied, the viscosity of these fluids at different temperatures is another important factor to be considered. The viscosity is a measure of the strength of a fluid to flow it is typically analyzed in terms of kinematic viscosity. At low viscosities, the dielectric fluid circulates or flows better inside the appliance and thus allows better heat dissipation.
Among the dielectric fluids used in electric apparatuses and having these and other properties, those dielectric fluids are included based on mineral oils, silicones, synthetic oils, vegetable oils with antioxidants or mixtures thereof.
Dielectric fluids based on mineral oils derived from petroleum, silicone-based oils or synthetic oils have been widely used in electrical transformers, transmission cables and capacitors. Examples of these oils are found in U.S. Pat. No. 4,082,866, U.S. Pat. No. 4,206,066, U.S. Pat. No. 4,621,302, U.S. Pat. No. 5,017,733, U.S. Pat. No. 5,250,750, and U.S. Pat. No. 5,336,847.
Although these dielectric fluids show a good performance as an insulating means and heat dissipation when used in electrical apparatuses, they present a high risk of contaminating the environment, when an accident occurs in which the fluid is spilled, because they contain synthetic elements, which are considered toxic and not biodegradable.
These disadvantages, as well as global environmental trends have resulted in the establishment of more stringent governmental and environmental regulations, requiring the industry to offer ecologic products, that is, products with low impact to the environment.
In response to the former problem, the dielectric fluids based on mineral oils, silicone-based or synthetic, a recently explored alternative is the development of dielectric fluids based on edible oil seeds, that may be seeds of soybean, sunflower, safflower, rapeseed, castor, linseed, cotton, rice, corn, olives and grapes.
It is well known that vegetable oils are deficient in terms of their property of stability to oxidation, that is, react easily with oxygen favoring degradation of the fluid, unlike the type of dielectric mineral, silicone or synthetic oils.
Recently, it has been shown that it is possible to optimize the oxidative properties of dielectric fluids based on edible seeds through changes in the manufacturing processes of the oil, or through the incorporation of synthetic compounds that have the primary role of slowing reactions with the oxygen, and thus, facilitating their use in electrical apparatuses such as transformers. Some of the solutions of vegetable oil dielectrics are described in patent documents GB-609133, CA-2204273, U.S. Pat. No. 5,766,517, U.S. Pat. No. 5,949,017, U.S. Pat. No. 5,958,851, U.S. Pat. No. 6,037,537, U.S. Pat. No. 6,159,913, U.S. Pat. No. 6,184,459, U.S. Pat. No. 6,207,626, U.S. Pat. No. 6,245,726, U.S. Pat. No. 6,274,067, U.S. Pat. No. 6,280,659, U.S. Pat. No. 6,312,623, U.S. Pat. No. 6,340,658, U.S. Pat. No. 6,347,033, U.S. Pat. No. 6,352,655, U.S. Pat. No. 6,398,986, U.S. Pat. No. 6,485,659, U.S. Pat. No. 6,645,404, U.S. Pat. No. 6,726,857, U.S. Pat. No. 6,905,638, and U.S. Pat. No. 7,048,875.
Some of the vegetable dielectric oils that have been developed present in their compound a high content of oleic acid in order to strengthen the oxidative capacity of the oil, as described in U.S. Pat. No. 5,949,017, U.S. Pat. No. 6,274,067, U.S. Pat. No. 6,312,623, U.S. Pat. No. 6,645,404, and U.S. Pat. No. 7,048,875. However, like other vegetable dielectric fluids they incorporate antioxidants to their compound, and other synthetic additives to improve this property, as well as other characteristics of the oil.
It has been shown that the addition of antioxidants or synthetic additives to the current vegetable dielectric oils compensates for poor stability to natural oxidation within these fluids, however, from the ecological point of view this is not desirable, since the chemical composition of these additives may reduce the ability of biodegradation of the oil, besides damaging its non-toxic characteristic, it is known that some of these synthetic compounds present toxic characteristics.
Therefore, there is a clear need to provide a dielectric fluid from vegetable oils with suitable characteristics, not only in performance for its use in electrical apparatuses, but also in environmental issues through the incorporation of zero synthetic additives or external antioxidants in its composition.